Spectral analysis and ridge-regression of magnetic anomalies from the northern continental margin of the Yucatán Peninsula, Mexico PublicDeposited

Descriptions

Marine magnetic data from the northern Yucatan continental margin were used to
obtain the radially-averaged power-density spectra (RAPDS) of 34 grids, each with
dimensions of 128 by 128 km, overlapping 50 percent. Depths to the tops of three
magnetic horizons were estimated from the slopes of linear segments in the spectra. The
depth to the base of the magnetic crust was estimated using an implicit relationship between
the deepest depth estimation and the spectral peak position. The depth determinations agree
with the limited drillhole data available and show some consistency with seismic refraction
interpretations for the study area. Results of similar studies, based on the spectral analysis
of magnetic anomalies in different tectonic settings, seem to agree with other geophysical
and geologic data. In this area however, the limited depth of resolution achieved by
seismic methods, and the absence of deep drillhole data and heat-flow measurements,
makes it difficult to assess the validity of this interpretation. I therefore examined the
simplifications implicit in the data processing techniques commonly applied in the spectral
analysis of magnetic anomalies, and the assumptions upon which the method is based.
Finally, I tested the method itself by interpreting synthetic anomalies generated from model
source bodies. I found that depth interpretations, based upon assuming that the slope of
linear segments in the RAPDS are proportional to the average depth to the top of magnetic
horizons, and thickness determinations using the position of the spectral peak, are
unreliable. The problem has been oversimplified. I also show that the RAPDS is
independent of the direction of both the geomagnetic field and the magnetization vector. It
can be represented by a Functional in terms of depth to the top of the source, its thickness
and its horizontal dimensions.
The problem of interpreting the RAPDS was then formulated as an inverse
problem. A solution was obtained through minimizing, iteratively, the sum of squares of
residuals between a real-data spectrum and a synthetic spectrum. The minimization was
based on a linearized model, using the ridge-regression algorithm. This technique
provides acceptable solutions for synthetic anomalies produced by model source bodies.
Depth and thickness determinations, obtained using this technique on the Yucatan spectra,
were used to make contour maps of the average depths to the top and the base of the
magnetic crust. The top of the magnetic crust is at an average depth of 1.8 km. The base
of the magnetic crust is at an average depth of 25 km. Beneath the central Campeche Bank
and Campeche Terrace the depth to the base is close to the depth to the crust-mantle
boundary determined by gravity modeling and mass column analysis. This suggests that in
those regions the mantle is the lower magnetic boundary.